3,6-Dichloropicolinic acid is disclosed in U.S. Pat. No. 3,317,549, as a highly active plant growth regulator which can be made by acid hydrolysis of 3,6-dichloro-2-(trichloromethyl)-pyridine.
In the approximately twelve years since the U.S. Pat. No. 3,317,549 patent issued, 3,6-dichloropicolinic acid (3,6-D, henceforth) has become increasingly of interest, particularly for control of weeds which can tolerate phenoxy-type herbicides--such as 2,4-D and MCPA. Thus, Canada thistle, Russian smartweed and wild buckwheat--for example--are susceptible to 3,6-D. The latter compound also has shown good activity on rangeland brush species, such as velvet mesquite, catclaw acacia and whitehorn. 3,6-D not only has a low toxicity to mammals, fish and birds, but is also relatively short lived in soil.
However, realization of the full potential of 3,6-D requires the development of a more efficient and economic method of synthesis than the preparation and hydrolysis of the corresponding trichloromethyl compound.
A substantially better method of preparing 3,6-D involves the reduction of 3,4,5,6-tetrachloropicolinic acid by successive reactions with hydrazine and an aqueous base, as described in U.S. Pat. No. 4,087,431. However, the efficiency of materials utilization in this process leaves something to be desired and the process cost is relatively high to be borne by an economic herbicide.
Another method which could be considered is electrolytic reduction of 3,4,5,6-tetrachloropicolinic acid. This of course would require replacement of both the 4 and 5 chlorines with hydrogen, under conditions such that hydrolysis (which is facile) and/or decarboxylation would not occur to any substantial extent. It would also require that over reduction, i.e., reduction of the 3,6-dichloropicolinic acid end-product--would not occur to a significant extend under the conditions required to effect the desired reduction.
U.S. Pat. No. 3,694,332 teaches that the 4-chlorine in tetrachloro-2-cyanopyridine can be replaced by hydrogen if the latter compound is co-dissolved with a neutral or acidic electrolyte (and water, as necessary) in an organic solvent and subjected to electrolytic reduction at a mercury (or lead) cathode. The patent also teaches that the same method may be used to reduce pentachloropyridine to 2,3,5,6-tetrachloropyridine (with coproduction of a small proportion of an unidentified trichloropyridine). The use of strong bases as electrolytes in the patented process is indicated as likely to result in hydrolysis reactions.
A co-pending application, Ser. No. 029,600, filed Apr. 13, 1979, in the name of D. Kyriacou as inventor, discloses that the electrolytic reduction of pentachloropyridine, co-dissolved with an aqueous base in an organic solvent and at a highly specific silver cathode, results in the replacement of the 4-chlorine, and then the 6- (or 2-) chlorine, with hydrogen.
No more pertinent prior art then the foregoing U.S. Pat. No. 3,694,332 patent is known of and the teachings of the patent and the aforesaid application fail to suggest to those knowledgeable in the prior art that tetrachloro-2-picolinic acid ("tet-acid") can be electrolytically reduced to the 3,6-dichloro acid (3,6-D) or to a trichloro-acid which will yield 3,6-D upon reduction. It may also be noted that attempts to electrolytically reduce several other polychloro-pyridine carboxylic acids in aqueous base solutions have failed, even when a siver cathode of the above mentioned type was employed. Similarly, attempts to so reduce a variety of chlorinated benzoic acids and phenols also were unsuccessful.
3,5,6-Trichloropicolinic acid ("3,5,6-T") is also a known compound which has herbicidal activity but for which a really economic method of synthesis has not been published. 3,4,6-Trichloropicolinic acid ("3,4,6-T") is a new compound (m.p. 128.degree. C.) (not discovered by the present inventors). Due to the high reactivity of the 4-chlorine in polychloropyridine compounds, it has been very difficult to devise a practical method of making 3,4,6-T. The only method known to have previously been found gives a very poor yield of the compound (and does not involve .dbd.C--Cl reduction).